1. Field of the Invention
The present invention relates to a combustor for an industrial gas turbine or an aircraft jet engine that is equipped with an air flow rate distribution control mechanism.
2. Description of the Related Art
Industrial gas turbines, aircraft jet engines, and incinerators discharge a small amount of toxic emissions into the surrounding atmosphere. The toxic emissions include carbon monoxide and unburned hydrocarbons, but the main problem is associated with the emission of oxides of nitrogen (NOx).
The amount of generated oxides of nitrogen shows strong dependence on temperature, and the concentration of oxides of nitrogen in the emissions can be reduced by lowering the combustion temperature in the combustion. Further, by using a premixing combustion system in which a mixed gas is combusted that is obtained by mixing a fuel and an oxidizer in advance, it is possible to suppress the formation of local high-temperature regions and control the combustion temperature. Further, by using a fuel-lean premixing combustion in which a fuel is combusted with excess air, it is possible to lower the average combustion temperature.
However, in a combustor using lean premixing combustion, when a mixing ratio of a fuel and an oxidizer is changed, pressure fluctuations can occur that can lead to the destruction of the combustor, or a sufficient combustion reaction does not proceed and unburned fuel is discharged from the combustor. The increase in the emission of unburned components degrades fuel efficiency and is also impermissible from the standpoint of preventing environmental pollution. Thus, the problem associated with lean premixing combustion is that such combustion can be effectively used only in a narrow mixing ratio range.
In an industrial gas turbine or aircraft jet engine, the flow rate of air and flow rate of fuel that flow into a combustor are determined according to the operation state, and in a combustor using lean premixing combustion, a plurality of fuel injection valves are disposed in the combustor and the flow rate of fuel injected from the fuel injection valves is regulated so as to maintain the mixing ratio of fuel and air of the combustion region in an adequate range.
In order to maintain the mixing ratio of fuel and air of the combustion region in an adequate range and sustain stable flame, a combustor is used that has a pilot combustion unit using premixing combustion or diffusion combustion with a comparatively high concentration of fuel for forming stable flame and a main combustion unit with a comparatively low fuel concentration for conducting low-NOx combustion. A combustor, in which a pilot combustion unit 18 is located in a center and a main combustion unit 19 is disposed around the pilot combustion unit, as shown in FIG. 7, is known as one embodiment of combustors having a pilot combustion unit and a main combustion unit. In such a combustor, stable flame is formed in the pilot combustion unit located in the center, and a mixed gas of the main combustion unit, which can easily become unstable, can be combusted with comparatively high stability. However, in the operation conditions of industrial gas turbines and aircraft jet engines, the air flow rate or fuel flow rate vary within a wide range. Therefore, the flow rate control of air flowing into the combustion region is sometimes conducted to maintain the fuel concentration in the combustion region within an adequate range and perform more stable combustion in the combustor.
Examples of mechanisms that control the flow rate distribution of air flowing into the combustion include a mechanism that controls the flow rate distribution of air flowing into the combustion region and dilution region, as disclosed in Patent Document 1 (Japanese Patent Number 3116081 “AIR DISTRIBUTION CONTROL GAS TURBINE COMBUSTOR”), and a mechanism that controls the flow rate distribution of air flowing into the pilot combustion unit and main combustion unit, as disclosed in Patent Document 2 (Japanese Patent Publication Number 06-137558 “FLOW RATE DISTRIBUTION CONTROL MECHANISM FOR GAS TURBINE BURNING APPARATUS”). With both mechanisms, the air flow rate is changed by changing an air passage area with a mechanically operated part. This movable part is provided in a passage of high-temperature high-pressure air discharged from the compressor, and a problem associated therewith is that the mechanism of the movable part is complex due to measures taken to prevent the effect of thermal expansion. Further, in aircraft jet engines, weight and size reduction are especially important, and because the addition of a complex mechanism increases weight, it is necessary to realize the air flow rate distribution control with as simple a mechanism as possible.